Service Engine Soon?

Now, I bought this Buick Lesabre 1993 from the newspaper and I drove it home and everything. I had to replace to alternator but other than that it was fine. A couple of days ago, when I was driving in overdrive, around 40 to 50 mph the car started to hesitate and vibrate a little. I just punched it and grew my speed up to 75 mph. After about 1 hour or so the Service Engine Soon light came on, so I proceeded to hit the brakes and as soon as it started to slow down the light went off. Now this keeps happening almost frequently. The car is an automatic. Does anyone have any ideas what is causing this? The car has had an oil change on 2/3/2005

You will need to download the malfunction code in the computer as to why the fault light came on. The procedure involves using a jumper wire to connect two pinouts in a certain connector. The connector is located under the radio/ash tray and easily accessed from under the dash.

The following link goes indepth on the procedure and the meanings of what the computer codes mean.

http://www.geocities.com/dann8785/serviceenginesoon.html